We're Green, and we're not going to take it any more

Greens leader Christine Milne is not very happy with the Prime Minister.

Kevin Rudd has spent the past week neutralising as many of Labor's political weaknesses as he can, and his announcements on both the carbon tax and asylum seekers have angered the Greens. The question is, what are the Greens going to do about it?

Milne says that Labor's move to bring forward a floating carbon price, or ETS, by one year is a "great disappointment" and "only the Greens can be trusted on climate". The announcement that all asylum seekers who arrive by boat will now be sent to Papua New Guinea and will never be eligible for settlement in Australia is a "radical, right-wing, hard-line response" and "a day of national shame".

The Greens have made it clear they will oppose both policies if Parliament returns before the election. Whilst the Coalition would probably support the PNG plan, Greens opposition to the carbon price package would kill it in the Senate. That's a good enough reason, if Rudd is looking for one, to go to an election in August rather than September.

Meanwhile, Milne and the Greens have a tough decision of their own to make. The Greens often refer to the 'old parties' and campaign, mostly in Labor-held seats, on the basis that both Labor and the Liberals amount to the same thing in the end. And on asylum seeker policy it's a valid argument. The shift in tone and intent between the PNG solution Rudd announced last week and his own Government's dismantling of offshore processing in 2008 is astonishing.

In practice, however, the Greens have always shown a clear preference for Labor over Liberal governments. The Federal Greens never considered supporting Tony Abbott after the 2010 election. In Tasmania and the ACT Greens MPs are also keeping Labor minority governments in power. Nobody really believed during party negotiations to form those governments that the Greens would back the Liberals. After Rudd defeated Julia Gillard for the Labor leadership, Milne immediately announced the Greens would "do nothing to facilitate a Tony Abbott Government".

Milne herself is the only Greens leader to have bucked this trend. In 1996, Milne was leader of the Tasmanian Greens when they won the balance of power in state parliament. The Labor Party refused to govern with Greens support, leaving Milne with no option but to back the Liberals. It was a bitter experience. Premier Tony Rundle ran to an early election and lost, but not before making a deal with Labor to reduce the size of Parliament and make it harder for the Greens to get elected. Milne lost her own seat in the 1998 election as a result.

It might just be time, though, for the Greens to put up or shut up. With a return to Parliament looking unlikely, the Greens can't vote Rudd out of office even if they wanted to. What they can do is not direct preferences to the Labor Party. Already the Greens leave it up to local branches to decide whether to support Labor candidates over the Liberals for House of Representatives seats. In the Senate, the Greens made a preference deal with Labor in 2010, and chances are they will again in 2010.

The Greens could show their disdain for both major parties by adopting a policy of preferencing all sitting Labor and Coalition MPs last in the House of Representatives. They could do a similar thing in the Senate by issuing split group voting tickets. Senate preferences are hugely important because over 95% of Australian voters cast above the line ballots. This means it's the party they vote for, and not the voter themselves, who decide where their preferences end up. By issuing separate group voting tickets directing half their votes to Labor and half to the Coalition, the Greens could effectively cancel out their preferences between the major parties.

Such a choice would send a powerful signal to Labor that the Greens couldn't be taken for granted. It wouldn't come without significant potential costs for the Greens. ALP preferences contributed to Greens Senators winning seats in Queensland and Western Australia in 2010, and Labor would certainly retaliate by directing their own preferences to another party. Labor did exactly that in 2004 when their preferences helped Family First's Steve Fielding get elected to the Senate with less than 2% of the vote. Further, sending some preferences to the Coalition risks helping Abbott to win control of the Senate, which is obviously not in the Greens's own interests.

The question the Greens need to answer, though, is this. If even Labor's new asylum seeker policy isn't enough for them to stop treating Labor as the lesser of two evils, what possibly could be? If they don't send a message now, Labor could be forgiven for thinking they never, ever will.